Method of forming a riveted joint

ABSTRACT

According to a first aspect of the application, there as provided a method of forming a riveted joint comprising a UHSS layer and a non-UHSS layer using a self-piercing rivet comprising a head and a shank, an outer diameter of a top of the shank being greater than an outer diameter of a bottom of the shank before insertion of the rivet, the method comprising pushing the self-piercing rivet into the UHSS layer such that the lower portion of the shank flares outwards and cuts a slug from the UHSS, and pushing the flared shank of the self-piercing rivet and the slug into the non-UHSS layer such that the non-UHSS layer deforms into a die recess and flows around an outer surface of the flared shank.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.17/265,063, filed Feb. 3, 2021, which is a U.S. National PhaseApplication of PCT GB2019/052193, filed Aug. 5, 2019, which claimspriority to United Kingdom Application No. 1812686.2, filed Aug. 3,2018, the entire contents of all of which are herein incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to a method of joining a workpiece whichcomprises Ultra High Strength Steel (UHSS), and also relates toassociated rivets. The method may be used to join a sheet of UHSS to anon-UHSS sheet (e.g. a sheet of softer more ductile material such asaluminium).

Self-piercing rivets are a well-established technology and are used, incombination with adhesives, to join metal work pieces together.Self-piercing rivets are used in the automotive industry to joinaluminium vehicle panels together. Aluminium is particularly suited tojoining using self-piercing rivets because it is relatively soft andrelatively ductile. In a typical joint formed using a self-piercingrivet, two overlapping sheets of material are positioned over a suitablyshaped die. A self-piercing rivet is inserted into the sheets ofmaterial. A shank of the rivet passes through the upper sheet ofmaterial and then flares outwardly as it travels into the lower sheet ofmaterial and approaches the die. When the shank flares outwardly itextends into the lower sheet such that material of the lower sheetencapsulates the flared portion of the shank. This provides a secureengagement between the rivet and the lower sheet of material. The upperand lower sheets of material are securely held between a head of therivet and the flared shank of the rivets, thereby forming a secure jointof the material. During rivet insertion the lower sheet material flowsinto the die and forms a so-called button (a raised disk) of material.Joining sheets of aluminium in this manner is efficient and effective,and is used extensively in the automotive industry during constructionof aluminium bodied vehicles.

Although self-piercing rivets are very effective in joining relativelysoft and ductile materials such as aluminium, they are less suited tojoining harder and less ductile material such as Ultra High StrengthSteel (UHSS). UHSS may be considered to mean steel with a tensilestrength of 800 MPa or more. Forming a joint in a workpiece thatcomprises UHSS using a self-piercing rivet is challenging. This isbecause the tensile strength of the UHSS is such that it is difficultfor the rivet to penetrate through the UHSS. The shank of the rivet willtend to collapse when an attempt is made to push it into the UHSS. Toavoid this problem the wall of the shank may be made thicker in order toincrease the column strength of the shank. This will allow the shank topenetrate the UHSS without buckling. However, if the shank of the rivetis thicker it will flare less when it is inserted into the workpiece. Asa result, there may be insufficient engagement between the rivet and alowermost sheet of the workpiece.

A further problem associated with joining a workpiece comprising a UHSSsheet, particularly if the rivet has an increased shank wall thickness,is that it is difficult to accommodate a slug of UHSS within the rivetbore. The UHSS slug may instead remain beneath the shank of the rivet,and may be pushed downwards into the workpiece by the rivet. The UHSSslug may compress a lowermost sheet of a joint until it that sheetbecomes too thin, and in some instances the UHSS slug may cut throughthe lowermost surface of the join. These issues may result in a poorquality join.

An additional or alternative way of avoiding rivet shank collapse whenjoining a workpiece that comprises a UHSS sheet is to use a rivet with avery high hardness (e.g. a hardness of 530 HV or more). This allows therivet to penetrate into the UHSS without collapsing. However, rivetswith very high hardness are relatively brittle and often exhibit crackswhen they have been inserted into a workpiece. This results in a jointwhich is relatively weak and may be prone to failure when the joint issubjected to a dynamic load (as may be experienced for example when avehicle is moving).

It is an object of the present invention to provide a method joining aworkpiece which overcomes a disadvantage associated with the prior art.

SUMMARY

According to a first aspect of the invention there as provided a methodof forming a riveted joint comprising a UHSS layer and a non-UHSS layerusing a self-piercing rivet comprising a head and a shank, an outerdiameter of a top of the shank being greater than an outer diameter of abottom of the shank before insertion of the rivet, the method comprisingpushing the self-piercing rivet into the UHSS layer such that the lowerportion of the shank flares outwards and cuts a slug from the UHSS, andpushing the flared shank of the self-piercing rivet and the slug intothe non-UHSS layer such that the non-UHSS layer deforms into a dierecess and flows around an outer surface of the flared shank.

By flowing the non-UHSS layer around the outer surface of the flaredshank, the invention is able to form a strong joint despite the factthat the shank is already flared when it enters the non-UHSS layer. Thisis in contrast to conventional self-piercing rivet joining methods,which rely upon the shank flaring when it enters a lowermost layer of aworkpiece.

A hole cut from the UHSS may have a diameter which is greater the outerdiameter of the bottom of the shank before insertion of the rivet. Theouter diameter of the top of the shank may be sufficiently large toprovide contact with the UHSS layer around a circumference of the shank.This is advantageous because when there is contact between the rivetshank and the UHSS layer around the circumference of the shank, thecontact prevents moisture ingress into the joint. Moisture ingress isundesirable because the moisture may cause corrosion.

The die may have a diameter which is no more than 2.5 mm greater thanthe outer diameter of the bottom of the shank before insertion of therivet. This small difference of diameter helps to flow the non-UHSSlayer around the outer surface of the flared shank.

The die may have a diameter which is at least 1.5 mm greater than theouter diameter of the bottom of the shank before insertion of the rivet.

The outer diameter of a top of the shank may be up to 12% larger thanthe outer diameter of the bottom of the shank.

The shank may comprise an upper portion and a lower portion, the upperportion having a greater diameter than the lower portion. A transitionfrom the upper portion to the lower portion may be tapered. The tapermay allow the upper portion of the rivet to enter the UHSS layer withoutexcessive force being required (if a step were present then excessiveforce might be needed).

The shank may comprise an upper portion and a lower portion, and whereinthe upper portion of the shank is tapered.

The shank may be tapered from the top of the shank to the bottom of theshank.

The taper may help to ensure that there is contact between the rivetshank and the UHSS layer around the circumference of the shank.

The die may include an annular ridge which extends around a cavity ofthe die.

The annular ridge may include a tapered face which is generally directedtowards the rivet.

According to a second aspect of the invention there is provided a methodof forming a riveted joint comprising a UHSS layer and a non-UHSS layer,using a self-piercing rivet comprising a head and a shank, the shankcomprising a lower shank portion, a middle shank portion and an uppershank portion, the middle shank portion having a diameter which is lessthan the diameter of the upper shank portion and the lower shankportion, the method comprising pushing the self-piercing rivet into theUHSS layer such that the lower portion of the rivet cuts a slug from theUHSS, and pushing the self-piercing rivet and slug into the non-UHSSlayer such the lower portion of the shank cuts a slug from the non-UHSSlayer, wherein a die receives the slugs of UHSS and non-UHSS, andwherein the die promotes flow of the non-UHSS around an outer surface ofthe lower shank portion.

By flowing the non-UHSS around the outer surface of the lower shankportion, the invention is able to form a strong joint even if little orno flaring of the shank occurs.

The die may comprise an annular ridge which extends around a cavity ofthe die.

The annular ridge may include a tapered face which is generally directedtowards the rivet.

The die may have a cylindrical opening with a diameter which is between0.5 mm and 1 mm greater than the diameter of the lower shank portion.

Insertion of the rivet into the UHSS may cause the shank of the rivet toflare outwardly.

According to a third aspect of the invention there is provided a methodof forming a riveted joint comprising a UHSS layer and a non-UHSS layer,using a self-piercing rivet comprising a head and a shank, the shankcomprising a lower shank portion, and an upper shank portion, a recessextending at least part way into the lower shank portion, wherein anouter diameter of the upper shank portion is greater than an outerdiameter of the lower shank portion, the method comprising pushing theself-piercing rivet into the UHSS layer such that the lower portion ofthe shank flares outwardly and cuts a slug from the UHSS, and pushingthe self-piercing rivet and slug into the non-UHSS layer such the lowerportion of the shank cuts a slug from the non-UHSS layer, wherein a diereceives the slugs of UHSS and non-UHSS, and wherein the die promotesflow of the non-UHSS around an outer surface of the lower shank portion,wherein the upper portion of the shank comes into contact with the UHSSlayer around a circumference of the shank.

An advantage of the third aspect of the invention is that it isrelatively easy to fabricate the self-piercing rivet. Removing alowermost portion of the outside of the shank and forming a recess inthe shank is relatively straightforward, compared for example withremoving a middle potion of the outside of the shank.

The die may comprise an annular ridge which extends around a cavity ofthe die.

The annular ridge may include a tapered face which is generally directedtowards the rivet.

The die may have a cylindrical opening with a diameter which is between0.5 mm and 1 mm greater than the diameter of the lower shank portion.

According to a fourth aspect of the invention there is provided avehicle comprising a riveted joint formed according to the method of anypreceding claim.

According to a fifth embodiment of the invention there is provided aself-piercing rivet comprising a head and a shank, an outer diameter ofa top of the shank being greater than an outer diameter of a bottom ofthe shank before insertion of the rivet.

The shank may comprise an upper portion and a lower portion. An outerdiameter of the upper portion of the shank may be greater than an outerdiameter of the lower portion of the shank.

A recess may be provided in the shank.

The recess may be provided in the lower portion of the shank.

The recess may have a depth which is less than half of the length of therivet. The recess may have a depth which is less than quarter of thelength of the rivet.

A tapered transition may extend between the upper portion and the lowerportion of the shank.

The outer diameter at the top of the shank may be up to 12% larger thanthe outer diameter of the bottom of the shank.

The outer diameter of the upper portion of the shank may be up to 12%larger than the outer diameter of the lower portion of the shank.

A lowermost portion of the recess may be tapered.

The recess may include a cylindrical portion.

The cylindrical portion of the recess in the shank may define acylindrical wall portion of the shank. A shank wall thickness of thecylindrical shank wall portion may have a ratio of between 0.16 and 0.3relative to the outer diameter of the shank at the cylindrical shankwall portion.

The cylindrical shank wall portion may have a shank wall thickness inthe range 1.0 mm to 1.5 mm.

The shank may have an outer diameter which is tapered from the top ofthe shank to the bottom of the shank.

The shank of the rivet may have a shank wall thickness in the range 1.0mm to 1.5 mm.

According to a sixth aspect of the invention there is provided a rivetedjoint comprising a UHSS layer and a non-UHSS layer formed using aself-piercing rivet according to the fifth aspect of the invention,wherein a lower portion of a shank of the rivet has flared outwards andis interlocked in the non-UHSS layer, and wherein an upper portion ofthe shank is in contact with the UHSS layer around a circumference ofthe shank.

The non-UHSS layer may form a layer below a slug of UHSS and encloses alower surface of the joint.

According to a seventh aspect of the invention there is provided aself-piercing rivet for forming a joint between a UHSS layer and anon-UHSS layer, wherein the self-piercing rivet comprises a head and ashank, the shank comprising a lower shank portion, a middle shankportion and an upper shank portion, and wherein the middle shank portionhas a diameter which is less than the diameter of the upper shankportion and the lower shank portion.

A maximum diameter of the upper shank portion may be the same as thediameter of the lower shank portion.

The middle shank portion may taper inwardly. The upper shank portion maytaper outwardly.

The middle shank portion may be generally cylindrical.

According to an eighth aspect of the invention there is provided ariveted joint comprising a UHSS layer and a non-UHSS layer formed usinga self-piercing rivet according to the seventh aspect of the invention,wherein a lower portion of a shank of the rivet is interlocked in thenon-UHSS layer, wherein an upper portion of the shank is in contact withthe UHSS layer around a circumference of the shank, and wherein a slughas been cut from the UHSS layer and a slug has been cut from thenon-UHSS layer, and wherein both slugs have been removed from the jointsuch that a bottom end of the shank is exposed.

According to a ninth aspect of the invention there is provided aself-piercing rivet for forming a joint between a UHSS layer and anon-UHSS layer, wherein the self-piercing rivet comprises a head and ashank, the shank comprising a lower shank portion, and an upper shankportion, a recess extending at least part way into the lower shankportion, wherein an outer diameter of the upper shank portion is greaterthan an outer diameter of the lower shank portion.

The outer diameter of the upper shank portion may be up to 12% largerthan the outer diameter of the lower shank portion.

The recess may be substantially conical.

The upper shank portion may meet the lower shank portion at atransition, the transition being tapered (e.g. chamfered or rounded).

According to a tenth aspect of the invention there is provided a rivetedjoint comprising a UHSS layer and a non-UHSS layer formed using aself-piercing rivet according to the ninth aspect of the invention,wherein a lower portion of a shank of the rivet has flared outwards andis interlocked in the non-UHSS layer, wherein an upper portion of theshank is in contact with the UHSS layer around a circumference of theshank, and wherein a slug has been cut from the UHSS layer and a slughas been cut from the non-UHSS layer, and wherein both slugs have beenremoved from the joint such that a bottom end of the shank is exposed.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 depicts in cross-section a rivet according to an embodiment ofthe invention;

FIG. 2 depicts in part of a rivet according to an alternative embodimentof the invention;

FIGS. 3A and 3B depict in cross-section forming a joint between twosheets of material made using a rivet according to an embodiment of theinvention;

FIG. 4 depicts part of a rivet according to an alternative embodiment ofthe invention;

FIG. 5 depicts in cross-section a joint formed using a rivet accordingto an alternative embodiment of the invention;

FIG. 6 depicts in cross-section a rivet according to an alternativeembodiment of the invention;

FIG. 7 depicts in cross-section an alternative joint between two sheetsof material made using a rivet according to an embodiment of theinvention;

FIG. 8 depicts in cross-section half of a rivet according to analternative embodiment of the invention;

FIG. 9 depicts in cross-section half of a rivet according to analternative embodiment of the invention;

FIG. 10A depicts in cross-section half of a rivet according to analternative embodiment of the invention partway through manufacture ofthe rivet;

FIG. 10B depicts in cross-section half of the rivet of FIG. 10A oncemanufacture of the rivet has been completed;

FIG. 11 depicts in cross-section a rivet according to an alternativeembodiment of the invention;

FIG. 12 depicts in cross-section an alternative joint between threesheets of material made using a rivet according to an embodiment of theinvention; and

FIG. 13 depicts in cross-section a die used to form the joint depictedin FIG. 12 .

DETAILED DESCRIPTION

A rivet 1 according to an embodiment of the invention is depicted inFIG. 1 . The rivet comprises a head 2 and shank 4. In a conventionalrivet the shank has a cylindrical outer surface which extends from abottom end of the shank to a top end of the shank where the shank meetsthe head (a radiused connection or a chamfer may be provided between theshank and the head). In contrast to this, in the rivet depicted in FIG.1 the shank has a lower portion 6 with a first diameter and an upperportion 8 with a second diameter which is larger than the diameter ofthe lower portion. The diameter of the upper portion 8 may for examplebe between 1 and 12% greater than the diameter of the lower portion 6.As is explained in more detail further below, the increased diameter ofthe upper portion 8 may correspond with flaring of the lower portion 6of the shank 4 which occurs when the rivet penetrates through an upperUHSS sheet. The hole cut into the UHSS sheet by the flared lower portion6 of the shank 4 may have a diameter which is 1-12% larger than thediameter of the lower portion of the shank before rivet insertion. Theincreased diameter upper portion 8 of the rivet shank will fill the holewhich was cut by the flared lower portion 6 of the shank 4. Thisprovides better engagement between the inserted rivet and the upper UHSSsheet than would otherwise be the case. In particular, it avoids thecreation of a gap between the rivet shank and the UHSS sheet whichextends from the head downwardly along the rivet shank. Such a gap isundesirable because a gap provides a readily accessible path formoisture ingress and a potential for subsequent galvanic corrosion. Inaddition, the presence of the gap will significantly impair the fatiguelife of the joint

A bottom end of the rivet 1 is provided with a recess 10 (which mayalternatively be referred to as a rivet bore). The recess 10 extendsupwards into the lower portion 6 of the shank. The recess 10 does notextend as far as the upper portion 8 of the shank. The recess 10 mayhave a depth which is less than half of the length of the rivet 1. Therecess 10 is dimensioned such that some flaring of the lower portion 6of the shank occurs when the rivet is inserted into a UHSS workpiece.The recess may have the form depicted in FIG. 1 or may have some otherform. The recess 10 may comprise a chamfered 14 lower portion, acylindrical middle portion 12 and a conical upper portion 18.

The chamfered lower portion 14 may define an included angle H in therange 60-120 degrees. The chamfered lower portion 14 may for exampledefine an included angle of around 90 degrees (e.g. plus or minus 10degrees). The chamfered lower portion 14 may promote flaring of theshank 4. If the lower portion 14 of the shank 4 were to extend axiallydownwards instead of being chamfered, then there would be a risk that noflaring of the shank would take place, as a result of which there wouldbe no interlock of the rivet in the lower sheet of the joint. Such alack of flaring may be referred to as ‘staking’. Although the lowerportion 14 of the shank 4 should not extend axially downwards, it is notessential that the lower portion is chamfered. The lower portion 14 mayhave some other inwardly tapering form. For example, the lower portionmay have a curved inwardly tapering form (e.g. having a concave curve).

The cylindrical middle portion 12 connects the chamfered lower portion14 to the conical upper portion 18. In the depicted embodiment, theconical upper portion 18 subtends an angle of around 15 degrees relativeto a plane perpendicular to an axis A of the rivet. However, the conicalupper portion may subtend some other angle. The conical upper portion 18in the depicted embodiment joins the cylindrical middle portion 12 via afillet radius 20. The fillet radius 20 may for example have a radius ofaround 0.4 millimetres (e.g. between 0.2 millimetres and 0.6millimetres).

In other embodiments, the upper portion of the recess may have someother suitable shape. For example, the upper portion may besubstantially planar (e.g. perpendicular to the axis A of the rivet).For example, the upper portion may be domed (i.e. with a curved ratherthan conical surface). The upper portion may comprise a combination ofplanar, conical and domed portions. For example, the upper portion maycomprise a domed inner portion and an outer frusto-conical portion. Theupper portion may comprise a combination of conical portions withdifferent taper angles, or may comprise a combination of domed portionswith different curvatures. The upper portion may be axially symmetric.

A tip 16 of the shank 4 at a lowermost end of the chamfer may berounded, e.g. as depicted. As noted above, the chamfered lower portion14 of the recess 10 promotes flaring of the shank. Rounding of the tipmay provide some control over the degree to which the rivet shankflares. A larger radius of curvature at the tip 16 will reduce rivetshank flaring, whereas a smaller radius of curvature at the tip willincrease rivet shank flaring. The rounded tip 16 may have a radius ofcurvature which is between 0.05 mm and 0.3 mm. It may be difficult toprovide a radius of curvature below 0.05 mm when using conventionalrivet fabrication techniques. A radius of curvature greater than 0.3 mmmay cause insufficient flaring to occur. In general, the radius ofcurvature of the tip may be selected based upon a desired degree ofrivet flaring, taking into account tensile strength of workpiecematerials that will be joined.

In an alternative arrangement (not depicted), a foot of the shank maycomprise a flat surface which is substantially perpendicular to the axisA of the rivet. An outer edge of the flat surface will comprise a tipwhich may have a radius of curvature as described above.

The diameter of the recess 10 will determine the thickness T of a wallof the lower portion of the shank 4. The wall thickness will influencethe extent to which the lower portion 6 of the shank flares when therivet 1 is inserted into a workpiece. If the wall is too thin then theshank may buckle when the rivet is inserted into a UHSS sheet.Conversely however, If the wall is too thick no flaring or insufficientflaring of the lower portion 6 of the shank 4 will occur and as a resulta strong joint will not be formed.

In one embodiment, as depicted in FIG. 1 , the lower portion 6 of theshank 4 may have a diameter of 5.5 millimetres, and the diameter of thecylindrical portion of the recess 10 may be between 2.5 mm and 3.5 mm.This provides a shank wall thickness in the range 1.0 mm to 1.5 mm. Thisrange of wall thicknesses provides some flaring of the lower portion 6of the shank 4 without the shank suffering from buckling (for a widerange of material types, including UHSS).

In other embodiments the rivet shank may have a diameter between 5millimetres and 6 millimetres. The width of the wall of the cylindricalpart of the lower portion 6 may for example be in the range 1.0 mm to1.5 mm. In general, the ratio of wall thickness to shank diameter may bebetween 0.16 and 0.3.

On the outer surface of the shank 4 there is a transition 22 between thelower portion 6 and the upper portion 8 of the shank. This transition 22may have various forms. In FIG. 1 the transition 22 is in the form of achamfer. The chamfer forms an angle C with respect to a line which isparallel to the axis A of the rivet 1. The angle C may for example bebetween 20 and 45 degrees (e.g. around 30 degrees). At this range ofangles the upper portion 8 of the shank 4 will enter a pierced hole inan upper UHSS sheet more readily and at lower forces than if thetransition for example had an angle of 90 degrees. If the angle of thetransition was significantly less than 20 degrees then the upper shankof the rivet may offer less engagement with the upper UHSS sheet. Thisnot desirable because engagement with the upper UHSS sheet is preferred(poor engagement may adversely affect the impair the fatigue life of thejoint). A transition 22 with an angle of around 30 degrees (e.g. +1-5degrees) may be preferred because it is easier to forge than otherangles.

A transition 22 is depicted in an enlarged view in FIG. 2 . Thetransition is again in the form of a chamfer, although the chamfer 22has a larger angle C than the chamfer of FIG. 1 .

In an alternative embodiment (not depicted), the transition between thelower portion 6 and the upper portion 8 of the shank 4 may be rounded.

A fillet radius is provided between the upper portion 8 of the shank 4and the head 2. The fillet radius may for example be in the range 0.2 mmto 1.2 mm. A filet radius is desirable, because if no filet radius ispresent then there may be a tendency for the rivet to crack at theconnection between the shank and the head. Providing a filet radiusreduces the likelihood of rivet cracking, and the larger the radius themore effective it is in reducing the likelihood or rivet cracking.However, when inserting the rivet into workpieces with high tensilestrength such as UHSS a larger filet radius will significantly increasethe force needed to insert the rivet into the workpiece. A filet radiusin the range 0.2 mm to 1.2 mm provides a balance between these twocompeting aspects.

The head 2 may for example have a diameter of around 7.75 mm. This isadvantageous because it is a standard rivet head diameter, and thusallows the rivet to be used in already installed rivet insertion tools.The head 2 may alternatively have some other diameter. A bottom end ofthe shank may for example have an outer diameter of around 5.5 mm. Thebottom end of the shank may have a smaller outer diameter, e.g. as smallas 5 mm. If the bottom end of the shank were to have a smaller outerdiameter than 5 mm then this may make the bottom end of the shank tooweak to penetrate into UHSS without buckling. The bottom end of theshank may have a larger outer diameter, e.g. up to 7 mm. If the bottomend of the shank were to have a larger outer diameter than 7 mm then thetop end of the shank may have an outer diameter which is too close insize to the diameter of the rivet head. If the difference in diameterbetween the top end of the shank and the rivet head is less than around0.3 mm then this may not be sufficient to allow a good interlock to beformed between the rivet head and a workpiece. Therefore, the outerdiameter of the bottom end of the shank, and the increase of diameter ofthe shank, taken together may be such that the outer diameter at the topend of the shank does not exceed 7.4 mm

FIGS. 3A and 3B depict schematically forming a joint using the rivetdepicted in FIGS. 1 and 2 . The rivet 1 is inserted into a workpiececomprising a UHSS sheet 30 and an aluminium sheet 32 located over a die34. The rivet may be inserted using a rivet insertion tool as describedin U.S. Pat. No. 6,692,213, which is herein incorporated by reference.The die 34 comprises a body 35 provided with a generally cylindricalrecess 36 in its upper surface. The die may include a rod (not depicted)which extends from an opposite surface in order to allow the die to beheld securely in a die holder. Examples of dies are described in U.S.Pat. No. 8,671,726, which is herein incorporated by reference. Therecess 36 of the depicted die has a flat bottom surface 38. In someembodiments a non-flat bottom surface may be provided. However, a flatbottom surface is preferred because a protrusion such as a pip extendingfrom the bottom surface of the die may cause the lowermost sheet of ajoint to be too thin.

Prior to insertion of the rivet 1 into the sheets 30, 32, the sheets areflat with a lower surface of the bottom sheet 32 being in contact withan upper surface of the die 34. The recess 36 of the die is empty.

FIG. 3A depicts the rivet 1 after it has been inserted into the UHSSupper sheet 30. The rivet 1 has cut a slug 42 from the upper sheet. Thelower portion 6 of the shank flared outwardly when the rivet 1 cut intothe upper sheet 30. The chamfered lower portion 14 of the rivet 1 hasbeen flattened against the slug 42 (although an upper end of thechamfered lower portion 14 remains).

Because the rivet shank 4 flared outwardly when it cut into the UHSSupper sheet 30, the slug 42 of UHSS (and a hole 44 left behind by theslug) has a diameter which is greater than an unflared diameter of thelower portion 6 of the rivet shank. This flaring of the lower portion 6of the rivet shank 4 as it cuts into the upper sheet 30 goes against theconventional understanding of how a self-piercing rivet joint should bemade. The conventional understanding is that the rivet should not flareduring insertion through an upper material sheet, and should only flarewhen it enters a lower material sheet (the flaring being promoted by thedie). This is because flaring in the lower sheet provides an interlockwith the lower sheet (interlock with the upper sheet is provided by thehead of the rivet). However, the inventors have understood that flaringof the shank during passage through the upper sheet 30 may be allowed tooccur, whilst still forming a strong joint between the upper and lowersheets 30, 32.

FIG. 3B depicts the rivet after it has been inserted into the lowersheet 32, i.e. when the joint has been completed. After it travelledthrough the upper sheet 30, the lower portion 6 of the rivet entered thelower sheet 32. The slug 42, because it is formed from UHSS, did notdeform and travel up into the recess 10 of the rivet (as would occur ifthe upper sheet were formed from a ductile material such as aluminium).Instead, the slug passed downwards into the lower sheet 32. In thedepicted example the lower sheet is formed from aluminium. The lowersheet material (aluminium) is compressed between the slug 42 and thebottom surface 38 of the die recess 36. The lower sheet material flowedsideways and away from a zone between the slug 42 and the die 34. Thegeometry of the die recess 36 promoted flow of the lower sheet material32 toward the lower portion 6 of the shank. The geometry of the dierecess 36 is discussed in detail further below. The lower sheet material32 has come into contact with an inwardly tapering part 46 of the lowerportion 6 of the shank 4. This may be referred to as back-filling of aspace behind a maximum diameter part of the lower shank portion 6.Back-filling may be considered to mean radially inward movement of thelower sheet material such that the lower sheet material contacts aninwardly tapering part 46 of the rivet shank 4. This back-filling of theinwardly tapering part 46 advantageously provides an interlock betweenthe rivet shank 4 and the lower sheet 32. No flaring of the rivet shankinto the lower sheet 32 is required in order to form an interlock withthe lower sheet.

The upper portion of the shank 8 is sufficiently large that it is incontact with the UHSS sheet 30 around a circumference of the shank.

A button 39 of lower sheet material flow has been formed. The button 39has an outer surface which generally corresponds in shape with thegeometry of the die recess 36. However, the button 39 includes ridge 41which extends around the button where the button meets a planar part ofthe lower sheet 32. The ridge 41 is formed because some lower sheetmaterial is forced out of the die recess 36 as the rivet is insertedinto the lower sheet 32.

In the above described embodiment the shaft 4 of the rivet 1 comprises alower portion with a first diameter and an upper portion 8 with a seconddiameter which is larger than the diameter of the lower portion. In thatembodiment both the lower portion 6 and the upper portion 8 arecylindrical and have outer surfaces which are parallel with the axis Aof the rivet 1. In an alternative embodiment of the invention, the upperportion of the rivet may be tapered instead of being cylindrical. Anexample of the upper portion of such an embodiment is depicted in FIG. 4. From a comparison between FIGS. 2 and 4 , it may be seen that there ischamfered transition between the lower portion and upper portion.Instead, the upper portion 50 is tapered to meet the lower portion 6(the lower portion is not tapered). In the depicted example the angle Dof the taper, relative to a line parallel to an axis of the rivet, is1.3 degrees. The taper begins where the lower portion 6 ends and theupper portion 50 begins. The taper is such that the upper portion of theshank has a minimum diameter of 5.5 millimetres and has a maximumdiameter of 5.6 millimetres. The maximum diameter of the shank 4 isconsidered to be the diameter immediately before a fillet radius whichconnects the shank 4 to the head 2 of the rivet. In general, the angle Dof the taper may be such that it provides an increase of the diameter ofthe upper portion 50 of the shank of between 1% and 12%. This maycorrespond with an angle D of the taper of between around 0.5 degreesand around 18 degrees. If the angle was significantly smaller than 0.5degrees then an upper end of the shank upper portion 50 would not have asufficiently large diameter to fill a hole cut by the flared rivet inthe UHSS upper sheet of a joint. If the angle was significantly greaterthan 18 degrees then the upper end of the shank upper portion 50 wouldbe bigger than the hole cut by the flared rivet in the UHSS upper sheet.Pushing the upper end of the shank upper portion 50 into the UHSS uppersheet would then require the UHSS to be pushed outwards and may causedamage to the rivet.

In use, the rivet with the tapered upper portion 50 functions in asimilar manner to the rivet depicted in FIGS. 1-3 . FIG. 5 depicts therivet after it has been used to form a joint in an upper sheet 30 and alower sheet 32 (the upper sheet 30 being UHSS). As may be seen, an upperend 51 of the tapered upper portion 50 of the rivet shank 4 is incontact with the upper sheet 30 around a circumference of the shank.This forms a stronger joint between the upper sheet and the lower sheet32. The upper portion 50 of the rivet shank 4 is in contact with theupper sheet 30, and as a result a secure connection is provided betweenthe rivet shank and the upper sheet. This provides improved fatigue lifecompared with for example a joint in which there is a gap between therivet shank and the upper sheet (i.e. the rivet shank has little or nocontact with the upper sheet). Although there is a gap between a lowerend 52 of the tapered upper portion 50 of the rivet shank 4 and theupper sheet 30, this does not have a significant effect upon the fatiguelife of the joint. The connection between the upper end 51 of thetapered upper portion 50 and the upper sheet 30 is sufficient to providethe improved fatigue life. In addition, there is no opening between therivet shank 4 and the upper sheet 30 through which moisture could enterthe joint. Moisture ingress is undesirable because it may lead tosubsequent galvanic corrosion.

A button 39 of lower sheet material flow has been formed. The button 39has an outer surface which generally corresponds in shape with thegeometry of the die recess 36. In common with the embodiment depicted inFIG. 3B, the button 39 includes ridge 41 which extends around the buttonwhere the button meets a planar part of the lower sheet 32. The ridge 41is formed because some lower sheet material is forced out of the dierecess 36 as the rivet is inserted into the lower sheet 32.

An advantage of the embodiment of FIG. 5 compared with the embodiment ofFIGS. 1-3 is that because there is less contact between the rivet shankand the upper sheet of the workpiece, setting force needed to insert therivet into the workpiece may be reduced. This may be advantageous if theupper sheet of the workpiece has a particularly high tensile strength(e.g. more than 1200 HV and/or if the upper sheet of the workpiece isparticularly thick (e.g. 2.5 mm or more). In general, embodiments inwhich the outer surface of the rivet shank is tapered may require lessinsertion force than embodiments in which the shank has an upper portionwith a larger diameter than a lower portion. This is because there isless contact between the rivet shank and the upper sheet of theworkpiece.

A rivet 3 according to a further embodiment of the invention is depictedin FIG. 6 . As will be appreciated from a comparison of this rivet withthe rivet depicted in FIG. 1 , the rivet depicted in FIG. 6 includesmany of the features of the rivet depicted in FIG. 1 . For example, therecess 10 has the same configuration, the tip 16 has the sameconfiguration, and the head 2 has the same configuration. However, theouter surface of the shank 4 differs from that of the shank depicted inFIG. 1 . Specifically, instead of having a cylindrical lower portion andan upper portion with a larger diameter, the shank has a tapered surface60. The tapered surface 60 of the shank 4 extends from the tip of theshank to a fillet radius which connects the shank 4 to the head 2 of therivet. The tapered shank 60 has a taper angle D with respect to a lineparallel to an axis A of the rivet. The taper angle D may be selectedsuch that the diameter of a top end of the tapered shank is between 1and 12% greater than the diameter of a bottom end of the tapered shank.The tapered shank 60 may advantageously at least partially fill a holeformed in an upper UHSS sheet cut by a flared shank of the rivet 3. Thismay be understood by referring to FIG. 6 in combination with FIG. 5(although FIG. 5 depicts a partially tapered rivet after insertion, therivet has the same form as fa fully tapered rivet after insertion). A, ahole 44 cut in an upper sheet of UHSS 30 is larger than the unflareddiameter of the bottom end of the shank of the rivet 3 of FIG. 6 becausesome flaring of the shank 4 occurs when the rivet cuts through the uppersheet. The tapered shank 60 provides an upper end of the shank 4 with alarger diameter than the lower end (before rivet flaring), and thus anupper end of the shank is in contact with the UHSS sheet 30 around acircumference of the shank. This provides the advantages mentionedfurther above in relation to the partially tapered shank.

Filling of the hole 44 in the UHSS upper layer 30 by the tapered shank60 may be less complete than filling of the hole by a stepped shank ofthe type depicted in FIG. 1 . In some instances it may desirable to fillthe hole 44 as completely as possible, for example to maximise fatiguelife and minimise the potential for moisture ingress.

The recess in the rivet shank may have the form depicted in FIG. 6 ormay have some other form. The recess 10 may comprise a chamfered 14lower portion, a cylindrical middle portion 12 and a conical upperportion 18. The chamfered lower portion 14 may define an included angleH in the range 60-120 degrees. The chamfered lower portion 14 may forexample define an included angle of around 90 degrees (e.g. plus orminus degrees). As noted further above, the chamfered lower portion 14promotes flaring of the shank 4. The cylindrical middle portion 12connects the chamfered lower portion to the conical upper portion 18.

In the depicted embodiment, the conical upper portion 18 subtends anangle G of around degrees relative to a plane perpendicular to an axis Aof the rivet. However, the conical upper portion may subtend some otherangle. The conical upper portion 18 in the depicted embodiment joins thecylindrical middle portion 12 via a fillet radius 20. The fillet radius20 may for example have a radius of around 0.4 millimetres (e.g. between0.2 millimetres and 0.6 millimetres).

In other embodiments, the upper portion of the recess may have someother suitable shape. For example, the upper portion may besubstantially planar (e.g. perpendicular to the axis A of the rivet).For example, the upper portion may be domed (i.e. with a curved ratherthan conical surface). The upper portion may comprise a combination ofplanar, conical and domed portions. For example, the upper portion maycomprise a domed inner portion and an outer frusto-conical portion. Theupper portion may comprise a combination of conical portions withdifferent taper angles, or may comprise a combination of domed portionswith different curvatures. The upper portion may be axially symmetric.

A tip 16 of the shank 4 at a lowermost end of the chamfer may berounded, e.g. as depicted. As noted above, the chamfered lower portion14 of the recess 10 promotes flaring of the shank. Rounding of the tipmay provide some control over the degree to which the rivet shankflares. A larger radius of curvature at the tip 16 will reduce rivetshank flaring, whereas as smaller radius of curvature at the tip willincrease rivet shank flaring. The rounded tip 16 may have a radius ofcurvature which is between 0.05 mm and 0.3 mm. It may be difficult toprovide a radius of curvature below 0.05 mm when using conventionalrivet fabrication techniques. A radius of curvature greater than 0.3 mmmay cause insufficient flaring to occur. In general, the radius ofcurvature of the tip may be selected based upon a desired degree ofrivet flaring, taking into account tensile strength of workpiecematerials that will be joined.

In an alternative arrangement (not depicted), a foot of the shank maycomprise a flat surface which is substantially perpendicular to the axisA of the rivet. An outer edge of the flat surface will comprise a tipwhich may have a radius of curvature as described above.

The shank 4 may be considered to comprise a lower portion 62 and anupper portion 64. The lower portion 62 is the portion of the shank 4within which the recess 10 extends. The upper portion 64 is the portionof the shank 4 within which the recess 10 does not extend. The diameterof the recess 10, combination with the angle of the tapered shanksurface 60, will determine the thickness T of a wall of the lowerportion 62 of the shank 4. The wall thickness will influence the extentto which the lower portion 6 of the shank flares when the rivet 1 isinserted into a workpiece. If the wall is too thin then the shank maybuckle when the rivet is inserted into a UHSS sheet. Conversely however,If the wall is too thick no flaring or insufficient flaring of the lowerportion 6 of the shank 4 will occur and as a result a strong joint willnot be formed.

The shank 4 may have a minimum diameter of 5.0 mm and may have a maximumdiameter of 6.0 mm. The diameter of the cylindrical portion 12 of therecess 10 may be between 2.5 mm and 3.5 mm. This provides a shank wallthickness in the range 1.0 mm to 1.5 mm. This range of wall thicknessesprovides some flaring of the shank 4 without the shank suffering frombuckling (for a wide range of material types, including UHSS). Ingeneral, the ratio of wall thickness to shank diameter may be between0.16 and 0.3.

A rivet according to the invention experiences some flaring of a lowershank portion when it enter an upper sheet of UHSS. This cuts a hole inthe upper UHSS sheet which is larger than the unflared diameter of thelower portion of the shank. In embodiments of the invention the upperportion of the shank has a larger diameter than the unflared lowerportion of the shank, and thus more effectively fills that hole thanwould otherwise be the case. When the rivet enters a lower sheet of moreductile material the lower shank portion has already undergone someflaring. The rivet may be referred to as having a pre-flared lower shankportion. A die promotes radially inward flow of the lower sheet materialtowards the pre-flared lower shank portion. Lower sheet material thuscomes into contact with an inwardly tapering part of the rivet shank.This may be referred to as back-filling and provides an interlockbetween the rivet and the lower sheet. Engagement with the upper sheetis provided by the head of the rivet, the head having a diameter whichis larger than the diameter of the hole cut into the upper sheet. Thus,the upper sheet and lower sheet are compressed together by the flaredlower shank portion and the head, thereby providing a strong jointbetween the sheets. In addition, in the example depicted in FIGS. 3 and5 (and for other embodiments not depicted), the upper portion or upperend of the rivet shank engages with the upper sheet. This providescontact between the rivet shank and the UHSS layer around acircumference of the shank. Because contact is provided around thecircumference of the shank, no gap is present which could allowundesirable moisture ingress. The contact around the circumference ofthe shank also provides improved resistance to fatigue (improved fatiguelifetime).

The depth of the recess in the embodiments of FIGS. 1 and 6 is 2.25 mm.The total length of the rivet is 5.25 mm. However, in other embodimentsof the invention the recess of the rivet may have a different depth andmay have a different shape from that depicted in FIGS. 1 and 6 .

The recess promotes flaring of the rivet. The depth of the recess mayfor example be less than half of the length of the rivet. The depth ofthe recess may for example be up to 3 mm. The recess may have any depthsmaller than 3 mm. This recess introduces some weakness into the rivet,and thus the depth of the recess in a rivet may be selected based uponthe tensile strength and the depth of the UHSS upper layer into whichthe rivet will be inserted. For a thicker UHSS layer or a stronger UHSSlayer a shallower recess depth may be selected, and for a thinner UHSSlayer or a weaker UHSS layer a deeper recess depth may be selected. Adeeper recess may promote more flaring of the rivet shank, which in turnmay provide a stronger interlock between the rivet shank and a lowermostsheet of a workpiece. These competing requirements may be balancedagainst each other, for example by performing test rivet insertionsusing samples of the materials that will be joined in a productionenvironment.

In an embodiment, the recess may for example comprise a chamfer 14 (e.g.with an included angle of 60-120 degrees, preferably around 90degrees+/−10 degrees). The recess may comprise an inward taper. Thechamfered or tapered recess may terminate at an upper end of the recesswhich is formed by a flat surface that extends substantiallyperpendicular to an axis of the rivet. In other embodiments the upperend of the recess may be formed by a domed portion or may be formed by aconical portion. The conical portion may have a larger included anglethan the chamfered or tapered portion. The conical portion may have anincluded angle of 140 degrees or more.

In another embodiment an upper end of the recess may comprise acombination of two or more forms. For example, the upper end of therecess may comprise an inner domed portion which is surrounded by anouter flat portion or conical portion (e.g. with an included angle of140 degrees or more). The inner domed portion may for example have adiameter of less than 2 mm, e.g. around 1 mm. The depth from a tip ofthe shank to the outer conical or flat portion at the upper end of therecess may determine the depth of the chamfered or tapered portion ofthe rivet. This depth will thus have a substantial effect upon flaringof the shank of the rivet. The inner domed portion may have a lessereffect upon flaring of the shank of the rivet. The inner domed portionis one example of a more general central recess or bore with arelatively narrow diameter (e.g. 2 mm or less).

The rivet may have a length of 4.5 mm or more (less than 4.5 mm may notbe sufficient to provide an effective joint between sheets of aworkpiece). The rivet may for example have a length of up to 7 mm. Therivet may have a length in excess of 7 mm, e.g. up to 9 mm. Such longerrivets may be used when forming a joint in an unusually thick workpiece(a rivet length of up to 7 mm is sufficient for most workpieces).

The die 34 may have a recess 36 which has a volume which is less thanthe volume of a conventional die used in convention self-piercingriveting. For example, when inserting a conventional self-piercing witha 5.5 mm shank diameter into an upper sheet 30 of 1500 MPa UHSS with athickness of 2 mm and a lower sheet 32 of aluminium with a thickness of2.5 mm, a die having a diameter of 10.0 mm and a depth of 2.0 mm wouldbe used. This die may be referred to as a DF10-200 die, and provides adie volume of 160 mm³. However, when self-piercing rivet insertionaccording to the invention is used, the die may have a diameter of 8.0mm and may have a depth of 2.0 mm. This die may be referred to as aDF08-200 die, and provides a die volume of 103 mm³. This die has avolume which is 64% of the volume of the conventional die. The volume ofthe die recess is reduced by reducing the diameter of the die recess(the depth of the die recess may remain unchanged).

In general, when forming a conventional self-piercing rivet joint a diemay be selected which has a diameter that is at least 3 mm greater thanthe diameter of the rivet shank (as measured before rivet insertiontakes place). In embodiments of the invention, a die with a smallerdiameter may be selected. This is because the smaller diameter dieincreases the force with which material from the lowermost workpiece ispushed out of the die, and thus promotes more flow of material intospace behind the flared shank (which may be referred to asback-filling). In embodiments of the invention die may be selected whichhas a diameter that is no more than 2.5 mm greater than the diameter ofthe rivet shank (as measured at the lowermost end of the rivet shankbefore rivet insertion takes place). The die may have a diameter whichis no more than 2 mm greater than the diameter of the rivet shank. Thedie may have a diameter which is between 1.5 mm and 2.5 mm greater thanthe diameter of the rivet shank (e.g. around 2 mm). The die diameterwhich is selected may be influenced by the thickness of the lowermostsheet of the workpiece, with a larger die diameter being used for athicker workpiece (in order to allow the workpiece to be accommodated).In one example, the lowermost end of the rivet shank may have a diameterof 5.5 mm before rivet insertion. A die may be selected which has adiameter of 8 mm if the workpiece has a thick lowermost sheet (e.g. 2.5mm or more). A die may be selected which has a diameter of 7.5 mm if theworkpiece is not thick (e.g. less than 2.5 mm). A die may be selectedwhich has a diameter of 7 mm. However, selecting such a die may increasethe forces required to form the joint (material of the lowermost sheetmust travel through a narrow gap) without significantly improving theinterlock between the lowermost sheet and the rivet shank. The aboveexamples refer to die diameters which increment in size by 0.5 mm. Otherdie diameters may be used, e.g. with a diameter which is 7 mm and 8 mm).

In general, the volume of the die may for example be less than 80% ofthe volume of a conventional die, or less than 65% of the volume of aconventional die, and may be less than 50% of the volume of aconventional die.

In conventional self-piercing riveting the rivet shank flares outwardsas the rivet approaches the die. However, in embodiments of theinvention little or no flaring of the rivet shank occurs as the rivetapproaches the die. Instead, the slug 42 is pushed downwards by therivet. This downward force places the lower sheet material 32 undersignificant compression, and this causes lower sheet material to extrudeout from beneath the slug. Because the diameter of the die recess issmaller than a conventional die, the lower sheet material is undersignificant compression as it passes between the slug 42 and sides ofthe die. As the lower sheet material leaves the gap between the slug andthe sides of the die it is able to expand and flow radially. The lowersheet material flows radially inwards and radially outwards. As a resultof the radially inward flow of the lower sheet material, lower sheetmaterial flows towards and contacts the inwardly tapering part 46 of therivet shank 4. Thus, the lower sheet material 32 at least partiallyback-fills a void created by the pre-flared tip of the rivet passinginto the lower sheet. This provides a good interlock of the rivet withthe lower sheet 32 and thus provides a strong join.

A die 71 with an alternative configuration is depicted in FIG. 7 . Inthis configuration an annular ridge 70 extends around an outer edge of arecess 73 of the die. The annular ridge 70 has a chamfered inner face 72which is generally directed towards the rivet. An upper end of thechamfered face 72 is connected to an outer step 74 by a fillet radius76. The annular ridge 70 promotes flow of the lower sheet material 32radially inwards (towards the inwardly tapering part 46 of the rivetshank 4). In some embodiments where the die is provided with an annularridge 70, a lesser reduction of die diameter may be used than when theannular ridge is not present. The reduced die diameter providescompression of the lower sheet material which promotes back-fillingbehind the rivet shank. If the annular ridge 70 is present then it mayprovide at least some of this effect, and as such the same level ofreduction of the die diameter may not be needed. In an embodiment, thediameter of the die may be less than 90% of a conventional die, lessthan 80% of a conventional die or maybe less than 65% of a conventionaldie.

The height of the annular ridge 70 may be selected such that lower sheetmaterial which is forced past the annular ridge provides a reactionforce which pushes material radially inwards towards the inwardlytapering part 46 of the rivet shank 4. The annular ridge 70 has a heightwhich is selected such that it does not cut too far into the lower sheet32. For example, the height of the annular ridge may be less than aquarter of the thickness of the lower sheet 32 (e.g. may be less than0.6 mm). The height of the annular ridge 70 may depend on the thicknessof the lower sheet 32 (for a thicker lower sheet a higher ridge may beused). The height of the annular ridge 70 may for example be in therange 0.2 mm-1.5 mm.

The angle of the chamfered face 72 of the annular ridge 70 may dependupon the ductility of the lower sheet 32. A smaller angle may be used ifthe lower sheet 32 is more ductile, and a larger angle may be used ifthe lower sheet is less ductile. For a less ductile the angle of thechamfered face 72 is greater to allow the chamfered face to cut into thematerial without significantly increasing the force needed to form thejoint. In general, the chamfered face 72 promotes flow of lower sheetmaterial towards the inwardly tapering part 46 of the rivet shank 4. Insome instances a chamfered face may not be needed if the lower sheet ofmaterial is sufficiently ductile (this may be determined by performingtest rivet insertions). The chamfered face 72 may for example have anincluded angle of up to 90 degrees. In an embodiment the face mayinclude some curvature. In general the face may be tapered (e.g.chamfered or curved).

In the above described embodiments of the invention the lower portion ofthe shank 4 is configured such that it flares as it is inserted into theupper sheet (e.g. UHSS) of a workpiece to be joined. The flaring of thelower portion of the shank provides an inwardly tapering outer surfaceon the shank which receives lower sheet material (e.g. as depicted inFIG. 3 ), and this provides an interlock of the rivet in the lowersheet. As explained further above, the configuration of the die maypromote the flow of lower sheet material into the inwardly tapering partof the rivet shank. This may for example be through the die having asmaller diameter than a conventional die and/or through the use ofannular ridge or other feature of the die. In alternative embodiments ofthe invention, as now described, a rivet may be provided which has aninwardly tapering shank portion before insertion of the rivet into theworkpiece. In other words, the rivet may be designed such that flaringof the shank is not required in order to form the inwardly taperingportion of the shank, but instead the rivet is manufactured with theinwardly tapering portion already provided on the shank.

FIG. 8 depicts in cross section half of a rivet 101 according to anembodiment of the invention. The rivet 101 comprises a head 102 and ashank 104. Unlike previous embodiments, no recess is provided at abottom end of the shank 104 (although in other embodiments a recess maybe provided). Some flaring of the flaring of the shank will neverthelessoccur when the rivet is inserted into a workpiece, e.g. due to forceapplied by the workpiece upper layer against the rivet when the rivet ispushed into the workpiece upper layer. In some embodiments a shallowrecess may be provided in the rivet, but this may be non-functional inthe sense that it does not weaken the strength of the shank such thatsignificant flaring of the shank occurs when the rivet is inserted intoa workpiece.

In the embodiment of FIG. 8 the shank 104 of the rivet 101 comprises alower shank portion 106, a middle recessed shank portion 108, and anupper shank portion 110. The diameter of the lower shank portion 106 isthe same as the diameter of the upper shank portion 110. This isdesirable because the diameter of the lower shank portion 106 willdetermine the size of the hole cut in the upper sheet of material beingjoined, and the upper shank portion 110 will fill that hole if it hasthe same diameter. This avoids gaps occurring between the rivet and theupper sheet when the joint has been formed. The upper sheet may forexample be UHSS.

The middle recessed portion 108 comprises an inward taper 112 whichextends from the lower shank portion 106 upwards towards the head 102.The inward taper 112 may be referred to as a chamfer 112. The chamfer112 may have an angle E of at least 10 degrees relative to a lineperpendicular to an axis A of the rivet. The chamfer may for examplehave an angle of up to 60 degrees, and may for example have an anglewhich is preferably 45 degrees or less. If the angle of the chamfer wereto be less than 10 degrees then sufficient lower sheet material may notbe received in the middle recess portion 108 to achieve a stronginterlock with the lower sheet. If the angle is too large then it may bedifficult for the lower sheet material to enter the middle recessportion and press against the shank.

An upper end of the middle recessed portion 108 has a taper 114 whichextends outwardly towards the head 102 of the rivet. The angle of theupper taper 114 may be any suitable angle. The angle may be selected, incombination with the length of the upper shank portion 110 to ensurethat an apex 116 of the middle recess portion 108 is within the lowersheet when a joint is made, such that lower sheet material can readilydeform into the middle recess portion 108 and into the apex 116. If thelower sheet material deforms as far as the apex 116 then this provides amaximum interlock between the rivet 101 and the lower sheet. A joint maybe formed without the lower sheet deforming as far as the apex 116,although this may be less strong than a joint in which the lower sheethas deformed as far as the apex. The position of the apex 116 may beselected based upon knowledge of the thickness of the lower sheet intowhich the rivet will be inserted.

A further alternative embodiment is depicted in FIG. 9 . The rivet 121of this embodiment has a head 102 and a shank 104. In this embodimentthe shank comprises a lower shank portion 126, a middle inwardlytapering portion 128 and an upper outwardly tapering portion 130. Thediameter of the lower shank portion 126 will determine the size of thehole cut into the upper sheet when a joint is made (the upper sheet mayfor example be UHSS). An upper end of the outwardly tapering portion 130may have a diameter which corresponds with the diameter of the lowershank portion 126. Consequently, the upper end of the rivet shank mayfill an upper part of the hole formed in the part of the upper sheet.The taper of the outwardly tapering portion 130 may be such that a gapbetween the rivet and the upper sheet exists further into the hole inthe upper sheet. However, the engagement between the upper part of theoutwardly tapering portion 130 and the upper sheet may be sufficient toprovide a strong interlock with the upper sheet (in combination with theengagement provided by the head 102).

The inwardly tapering portion 128 may have a taper angle whichcorresponds with that described above in connection with FIG. 7 . Thatis the inwardly tapering portion 128 may have an angle F of at least 10degrees relative to a line perpendicular to an axis A of the rivet. Theinwardly tapering portion 128 may for example have an angle of up todegrees, and may for example have an angle which is preferably 45degrees or less.

An apex 132 between the inwardly tapering portion 128 and the outwardlytapering portion 130 has a position which is determined by the angles ofthe inwardly tapering and outwardly tapering portions. The position ofthe apex 132 may be selected such that it is within the lower sheetmaterial, thereby allowing the lower sheet material to be readilydeformed into the apex. The selection may be based upon knowledge of thethickness of the lower sheet into which the rivet will be inserted.

In the embodiments of FIGS. 7 and 8 the lower shank portion 106, 126 hasa length (in the direction parallel to the axis A of the rivet) which issufficiently large that the lower shank portion does not undergosignificant deformation during piercing through a UHSS sheet. If thelower shank portion 106, 126 were shorter then there would be a riskthat it would deform when piercing through a UHSS sheet (the UHSS mayhave a similar tensile strength to the rivet, and thus may causedeformation of the rivet) However, the length of the lower shank portion106, 126 is sufficiently small that the inwardly tapering portion 112,128 of the rivet will be received in lower sheet material such thatlower sheet material may flow into that portion. The length of the lowershank portion 106, 126 may for example be in the range 0.5 millimetresto 2 millimetres, and may be in the range 1 millimetre to 1.5millimetres.

The embodiments depicted in FIGS. 8 and 9 may be constructed by coldforging followed with a secondary rolling or machine operation to formthe tapered parts of the shank. An embodiment is depicted in FIG. 10which does not require secondary rolling or machining. Referring firstto FIG. 10A, a rivet 141 is formed which comprises a head and a shank104. The rivet does not have a recess at the bottom of the shank(although in other embodiments a recess may be provided). A step 140 iscut into the shank to form a lower shank portion 142 which has a reduceddiameter compared with an upper shank portion 143. The lower shankportion 142 may be less than 50% of the length of the shank 104, lessthan 40% and may preferably less than 30% of the length of the shank. Afurther manufacturing step is applied to the rivet, namely forging therivet tip 144 radially outwards such that, as depicted in FIG. 10B a lip146 is formed at the tip of the rivet. The lip 146 may have a diameterwhich corresponds with a diameter of the upper shank portion 143 of theshank 104. The step 140 and the lip 146 in combination define an annulargroove 150. The annular groove 150 may receive lower sheet material whena joint is formed in the manner described above in connection with otherembodiments.

FIG. 11 depicts a rivet 201 according to an alternative embodiment ofthe invention. A shank 204 of the rivet 201 has a lower portion 206 witha first diameter and an upper portion 208 with a second diameter whichis larger than the diameter of the lower portion. The diameter of theupper portion 208 may for example be between 1 and 12% greater than thediameter of the lower portion 206. The upper portion 208 may have anouter diameter of 5.5 mm. The lower portion 206 may have an outerdiameter of 5 mm. A transition between the upper and lower portion is inthe form of a concave connection 222. However, the transition may havesome other tapered form. For example a chamfered connection may beprovided. A 90 degree step is not preferred because it may be difficultto push such a step through UHSS.

The rivet 201 includes a recess 210 at a bottom end of the shank 204.The recess is conical in this embodiment, but may have some other shape.The recess 210 causes the rivet shank to flare when it is inserted intoan upper sheet of UHSS.

The lower portion 206 of the shank 204 may for example have a length inthe axial direction which is less than half of the length of the rivet201. The lower portion 206 of the shank 204 may for example have alength in the axial direction of around 2.5 mm. The recess 210 may havea depth which is less than half of the length of the rivet. The recess210 may have a depth which is less than half of the length of the lowerportion of the shank 204.

The rivet has a head 202, which may have a diameter of 7.75 mm. The head202 is connected to the shank upper portion 208 via a fillet radius 212.The fillet radius may for example be around 0.4 mm or more (it may bedifficult to cut a fillet radius which is smaller than this). The filletradius may for example be up to around 0.8 mm (if the fillet radius islarger than this then difficulties may arise pushing the rivet into theworkpiece (as explained further above). The fillet radius may be between0.5 mm and 0.75 mm. Using a small fillet radius in this range aidssealing of the rivet against a UHSS upper sheet and reduces thelikelihood of a gap being present between the rivet head and the UHSSupper sheet (see FIG. 12 as described below).

FIG. 12 depicts a joint made using the rivet 201 depicted in FIG. 11 .Unlike previously illustrated and described joints, this joint does notinclude a button of lower sheet material beneath the rivet. Instead, therivet penetrates through all sheets of material which form the joint,and a bottom end of the shank is exposed.

When the rivet 201 is inserted into an upper sheet of UHSS 230, thelower portion 206 of the shank 204 flares outwardly. As a result, aninwardly tapering portion 246 is formed in the outer surface of theshaft. A recess 248 is thereby formed by the combination of the inwardlytapering portion 246 and the rounded connection 222. Lower sheetmaterial flows into the recess 248, thereby providing an interlockbetween the rivet and the lower sheet 232.

In common with other embodiments, the increased diameter of the upperportion 208 may correspond with flaring of the lower portion 206 of theshank 204 which occurs when the rivet penetrates through an upper UHSSsheet. The hole cut into the UHSS sheet by the flared lower portion 206of the shank 204 may have a diameter which is 1-12% larger than thediameter of the lower portion of the shank before rivet insertion. Theincreased diameter upper portion 208 of the rivet shank fills the hole244 which was cut by the flared lower portion 206 of the shank 204 (asmay be seen in FIG. 9 ). This provides better engagement between theinserted rivet and the upper UHSS sheet than would otherwise be thecase. In particular, it avoids the creation of a gap between the rivetshank and the UHSS sheet which extends from the head downwardly alongthe rivet shank. Such a gap is undesirable because this may allowmoisture ingress into the joint and may impair the fatigue life of thejoint.

FIG. 13 depicts a die 250 which may be used together with the rivetdepicted in FIG. 11 . The die 250 comprises a cylindrical wall 252 whichdefines a cylindrical opening 253. Unlike dies depicted in precedingfigures, the die 250 does not include a bottom surface which isconfigured to deform the lower sheet to form a button of material.Instead, the die 250 is open at its bottom end 254. This allows slugs ofmaterial cut from sheets being joined to pass down and out of the die250. The diameter of the cylindrical opening 253 of the die 250 may beselected with reference to the diameter of the lower portion 206 of therivet 201 (as measured before rivet insertion). For example, if thelower portion 206 of the rivet 201 has a diameter of 5 mm then thecylindrical opening 253 of the die 250 may have a diameter of between5.8 mm and 6 mm. This is sufficiently large to receive slugs of materialcut from sheets being joined. In general, the lower portion of the rivetmay have a diameter between 4 mm and 6 mm. The cylindrical opening 253of a corresponding die 250 may have a diameter between 4.5 mm and 7 mm.The diameter of the cylindrical opening may be between 0.5 mm and 1 mmgreater than the diameter of the lower portion 206 of the rivet 201.

In common with the die described further above, the die 250 of FIG. 13includes an annular ridge 254 which extends around an outer edge of thecylindrical opening 253 of the die. The annular ridge 254 has aninwardly facing chamfered inner face 256. An upper end of the chamferedface 256 is connected to a flat upper surface 257 of the annular ridge254. A downward step 258 is provided at an outer edge of the annularridge 254. The annular ridge 254 promotes flow of lower sheet materialradially inwards when a join is being formed (towards the inwardlytapering portion 246 of the rivet shank 204).

The height of the annular ridge 254 may be selected such that lowersheet material which is forced past the annular ridge provides areaction force which pushes material radially inwards towards theinwardly tapering part 246 of the rivet shank 204. The annular ridge hasa height which is selected such that it does not cut too far into alower sheet of a joint. For example, the height of the annular ridge maybe less than a quarter of the thickness of the lower sheet (e.g. may beless than 0.6 mm). The height of the annular ridge 254 may depend on thethickness of the lower sheet being joined (for a thicker lower sheet ahigher ridge may be used). The height of the annular ridge 254 may forexample be in the range 0.2 mm-1.5 mm.

The annular ridge 254 includes a chamfered face 256. If there is somemisalignment between the rivet 201 and the die 250, then the rivet mayhit the chamfered face 256 and the chamfered face may guide the rivetinto the bore 253. In general, the annular ridge may include a taperedinner face. In some embodiments the annular ridge may not have a taperedinner face.

Referring to FIG. 12 it may be seen that a strong joint has been madebetween the UHSS upper sheet, the middle sheet and the lower sheet. Thelowermost sheet has flowed into the inwardly tapering portion 246 of therivet shank 204, and forms a strong interlock between the lowermostsheet and the rivet 201. The annular ridge 254 of the die has beenpressed into a bottom surface of the lowermost sheet, and has formed anannular recess in the lowermost sheet which surrounds the rivet.

Embodiments of the invention may be used to join UHSS to aluminium.Embodiments of the invention may be used to join UHSS to any othersuitable material. For example, embodiments of the invention may be usedto join UHSS to steel which has a lower tensile strength than UHSS.

The term UHSS as used in this document may refer to steel with a tensilestrength of MPa or more. The steel may have a tensile strength of 1000MPa or more, and may have a tensile strength of 1500 MPa or more. Thesteel may have a tensile strength of up to 2000 MPa.

A rivet according to an embodiment of the invention may be formed fromsteel. The steel may for example have a hardness of 4 or 5. This islower than a rivet steel hardness of which would conventionally be usedwhen forming a joint in UHSS. Prior art methods of joining a workpiecethat comprises a UHSS sheet use rivets having a hardness of 530 HV ormore. However, when using an embodiment of the invention a softer rivetmay be used. This is because the rivet has a recess which is not as deepas the recess in a conventional rivet, and thus does not weaken therivet to the same extent. For example, a rivet with a hardness of lessthan 530 HV may be used. For example, a rivet with a hardness of between400 HV and 510 HV may be used. In other embodiments, a rivet with ahardness of 510 HV or more may be used (e.g. a rivet with hardness of530 HV or more).

In this document the term “tapered” in connection with a diameter may beinterpreted as meaning a gradual transition from a larger diameter to asmaller diameter. It does not necessarily require that the transition islinear.

Different features of rivets according to different embodiments of theinvention may be combined together. For example, a recess shape or depthas described in one embodiment may be used for other embodiments. Forexample, a taper or step provided on the outer surface of the shank inone embodiment may be used for other embodiments. For example, ageometry provided at a lowermost end of a rivet shank in one embodimentmay be used for other embodiments.

In this document references to the diameter of the shank are intended toexclude a transition (e.g. fillet radius) between the shank and the headof the rivet.

1. A self-piercing rivet comprising: a head and a shank, a bottomportion shank comprising a recess; wherein the recess includes acylindrical portion that defines a cylindrical wall portion of theshank, and wherein a shank wall thickness of the cylindrical shank wallportion has a ratio of between 0.16 and 0.3 relative to the outerdiameter of the shank at the cylindrical shank wall portion.
 2. Theself-piercing rivet of claim 1, wherein the recess defines a recessdepth and the self-piercing rivet defines a rivet length, the recessdepth being less than half of the rivet length.
 3. The self-piercingrivet of claim 1, wherein the cylindrical wall portion has a thicknessin the range 1 mm to 1.5 mm.
 4. The self-piercing rivet of claim 3,wherein the diameter of the cylindrical portion of the recess is between2.5 mm and 3.5 mm.
 5. The self-piercing rivet of claim 1, wherein therecess comprises an inwardly tapered portion.
 6. The self-piercing rivetof claim 5, wherein the inwardly tapered portion is a chamfered portion.7. The self-piercing rivet of claim 6, wherein the chamfered portiondefines an included angle in the range 60 to 120 degrees.
 8. Theself-piercing rivet of claim 6, wherein the shank defines a tip that isdisposed at a lowermost end of the chamfered portion of the recess. 9.The self-piercing rivet of claim 8, wherein the tip comprises a roundedportion.
 10. The self-piercing rivet of claim 9, wherein the roundedportion of the tip is disposed at a lowermost end of the chamfered lowerportion of the bore.
 11. The self-piercing rivet of claim 9, wherein therounded portion of the tip defines a radius of between 0.05 mm and 0.3mm.
 12. The self-piercing rivet of claim 9, wherein the rounded portionof the tip adjoins the outer surface of the bottom portion of the shank.13. The self-piercing rivet of claim 1, further comprising a foot thatcomprises a flat surface that is substantially perpendicular to an axisof the rivet.
 14. The self-piercing rivet of claim 6, wherein the shankdefines a tip that is disposed at a lowermost end of the chamferedportion of the recess, and wherein the self-piercing rivet furthercomprises a foot that comprises a flat surface that is substantiallyperpendicular to an axis of the rivet, and wherein the tip is disposedat an outer edge of the flat surface.
 15. The self-piercing rivet ofclaim 1, wherein the chamfered portion terminates at an upper end of therecess, the upper end of the recess being domed, conical, or planar. 16.The self-piercing rivet of claim 1, wherein a lower portion of therecess has an inwardly tapering form.
 17. The self-piercing rivet ofclaim 16, wherein the lower portion has a curved inwardly tapering form.18. The self-piercing rivet of claim 16, wherein the lower portion ofthe recess has a concave form.
 19. A self-piercing rivet comprising: ahead and a shank, a bottom portion shank comprising a recess; whereinthe recess comprises an inwardly tapered portion, the inwardly taperedportion defining an included angle in the range 60 to 120 degrees.
 20. Aself-piercing rivet comprising: a head and a shank, a bottom portionshank comprising a recess; wherein the recess defines a cylindricalportion that defines a cylindrical wall portion, the cylindrical wallportion having a thickness in the range 1 to 1.5 mm.